The objectives of this investigation are to determine the mechanisms and the possible physiological significance of enzyme regulation by a diverse series of apparently distally related groups of intermediary metabolites. Phosphoenolpyruvate carboxylase of bacterial origin is subjected to this type regulation. Its primary function is to make C-4 compounds for the citric acid cycle from a C-3 compound originating in glycolysis. It is activated by a precursor of that C-3 compound and also by the cosubstrate of the citric acid cycle reaction in which its product is also a substrate. Aspartate, the transaminated product of this carboxylase reaction, is an inhibitor of its activity. Pyrimidine phosphates (particularly CDP), end-products of one of the metabolic pathways of aspartate utilization, are activators. This investigation will attempt to determine if the kinetics of interaction of each of these compounds in vivo follow the same mechanism as in vitro. Through the use of genetically altered enzymes, the structural requirements for these diverse interactions will be investigated also.